Hot-dip galvanization bath for parts made from any steel composition

ABSTRACT

Hot-dip galvanisation bath for parts made from any steel composition, in particular parts made from steel containing silicon and/or phosphorus, having undergone a prior degreasing, acid dipping and fluxing treatment, characterised in that it contains zinc as well as 0.1 to 1.5% by weight of bismuth and 0.1 to 1.5% by weight of tin.

The present invention relates to a hot-dip galvanisation bath for partsmade from any composition of steel, which may or may not contain siliconand/or phosphorus.

It is a known fact that parts made from iron, cast iron or steel have tobe protected against corrosion in all fields of industry.

Galvanisation, in particular hot galvanisation based on dipping, whichinvolves covering the parts to be protected with a zinc-based coating,is one of the most common processes used to obtain such protection.

To this end, the parts to be treated are dipped in a bath of molten zincor a zinc alloy at a temperature in the order of 400 to 500° C.

Before carrying out this operation, the parts to be treated must beprepared so that they are in a state ready to receive the galvanisedcoating, enabling this coating to be deposited uniformly on their entiresurface.

This initial treatment conventionally involves successive steps ofdegreasing, generally carried out using an alkaline medium, acid dippingusing a corrosion inhibitor and fluxing in a pre-treatment bath whichusually contains zinc chloride and ammonium chloride.

Between the degreasing, acid dipping and fluxing steps, the parts to betreated are usually rinsed with water.

In addition, a stoving step may be incorporated between thesepreliminary steps and the treatment step in the galvanisation bath,which involves drying the interface layer obtained at the end of thefluxing step by subjecting the parts to be treated to an increasedtemperature.

The galvanised coatings must be uniform, non-marbled and shiny inappearance, must exhibit good adhesion with respect to the steel andshould also have a homogeneous thickness in the order of 10 to 70 μm asa rule.

In order to improve these characteristics and obtain totallysatisfactory galvanised coatings, it has already been proposed thatother elements should be added to the molten zinc, such as nickel,copper, lead, iron, cobalt or alternatively aluminium, for example.

In particular, it is known that adding aluminium improves the sheen ofthe galvanised coatings, reduces superficial oxidation of the zinc,improves the fluidity of the bath and enables the zinc/iron reaction tobe controlled, which contributes to obtaining the requisite thickness.

However, whilst non-alloyed steels and malleable cast irons can besatisfactorily treated in conventional galvanisation baths, especiallythose containing aluminium, the same is not true of certain alloyedsteels, in particular steel sheets with high contents of silicon and/orphosphorus.

In the case of such steels, the conventional hot-dip galvanisationoperation in effect produces coatings which have a greyish appearancewhich is not satisfactory from an aesthetic point of view, an abnormallyhigh thickness which can be as much as 400 or even 500 μm and which alsoare not very adherent and not very resistant to impacts (risk of peelingunder the effect of localised impacts).

These problems are essentially attributable to the fact that thepresence of silicon and/or phosphorus increases the reactivity of thesteel and encourages the rapid formation of fragile inter-metalliccompounds.

In order to study this phenomenon in more detail, specialists definedthe concept of silicon equivalent for a steel (Si equivalent=Si+2.5P)and analysed the variations in the thickness of a galvanised coatingdeposited on a steel part as a function of the amount of Si equivalentcontained in this steel.

By plotting what is known as the Sandelin curve illustrated in FIG. 1,they were therefore able to establish that, in the case of steelscontaining silicon and/or phosphorus, the thickness of the galvanisedcoating is not a linear function of the content of Si equivalent.

In effect, the Sandelin curve is characterised by a thickness peak knownas the “Sandelin peak”, the presence of which proves that growth in thegalvanised coating is very rapid if the content of Si equivalent is inthe vicinity of 0.1%.

As a result, conventional hot-dip galvanisation baths enablesatisfactory results to be obtained for steel compositions known as“hypo-Sandelin steels” with a low content of silicon or siliconequivalent (Si equivalent<0.01%) but not for steel compositions with ahigh content of Si equivalent.

New steels have recently been developed, which are distinctive due to anot inconsiderable content of silicon and/or phosphorus, such as highelastic limit steels (HEL) or very high elastic limit steels (VHEL)which contain up to 2% of Si equivalent, the mechanical characteristicsof which are particularly interesting.

Accordingly, it would be of advantage to devise a hot-dip galvanisationbath of a nature that would enable satisfactory coatings to be obtainedin terms of appearance, adhesion and thickness for all steelcompositions, including steels with a very low Si equivalent content andHEL or VHEL steels, i.e. to be able to “smooth” the Sandelin curve.

The objective of the invention is to propose a hot-dip galvanisationbath for steel parts, based on a specific composition enabling thisobjective to be achieved.

Such a hot-dip galvanisation bath is adapted for the treatment of partsof any steel composition which have previously undergone a degreasing,acid dipping and fluxing pre-treatment.

It is characterised in that it contains zinc as well as 0.1 to 1.5% byweight of bismuth and 0.1 to 1.5% by weight of tin.

Surprisingly, it has been found that adding bismuth and tin to a hot-dipgalvanisation bath as proposed by the invention enables the fluidity ofthis bath to be improved, thereby promoting penetration of the coatingon the surface of the parts to be treated and hence their adhesion.

By virtue of a preferred characteristic of the invention, thegalvanisation bath additionally contains at last one metal selected fromthe group comprising vanadium, manganese and aluminium.

This bath may specifically contain 0.04 to 0.15% by weight of vanadiumand/or 0.10 to 0.30% by weight of manganese and/or at least 0.002% byweight of aluminium.

The rest of such a bath making up the 100% by weight comprises, inaddition to the bismuth and tin, zinc of commercial purity (Z1 or Z2quality with a minimum zinc content of 99.995% or 99.95% respectively).

It has been confirmed that, as a result of the invention, addingmanganese and/or vanadium and/or aluminium to the hot-dip galvanisationbath surprisingly reduces the reactivity of the zinc and hence thethicknesses of the coatings for a large range of steels with highcontents of silicon and/or phosphorus.

Such a bath enables steels which have only a low content of siliconand/or phosphorus to be treated satisfactorily and, at the same time,also makes it possible to produce coatings on such steels which satisfyrequirements in terms of aesthetic appearance, thickness and adhesion.

As a result of one characteristic of the invention, the hot-dipgalvanisation bath contains between 0.06 and 0.12% by weight ofvanadium.

As a result of another characteristic of the invention, the hot-dipgalvanisation bath contains between 0.15 and 0.25% by weight ofmanganese.

As a result of another characteristic of the invention, the hot-dipgalvanisation bath contains between 0.0040 and 0.020% by weight ofaluminium.

The hot-dip galvanisation bath proposed by the invention mayadvantageously contain, as a proportion by weight, between 0.5 and 1.5%of bismuth, between 0.5 and 1.5% of tin, between 0.06 and 0.12% ofvanadium, between 0.15 and 0.25% of manganese and between 0.004 and0.20% of aluminium.

It has proved possible to obtain, over a broad range of steelcompositions which may or may not have high contents of silicon and/orphosphorus, galvanised coatings that are totally satisfactory in termsof appearance, adhesion and thickness using a hot-dip galvanisation bathcontaining, as a proportion by weight: 0.10% of vanadium, 0.17% ofmanganese, 0.006% of aluminium, 0.2% of tin and 0.2% of bismuth, therest making up the 100% being zinc of commercial purity.

It has also been established that using a galvanisation bath proposed bythe invention enables mechanical characteristics to be improved and inparticular the fatigue resistance of HEL and VHEL steels.

It is a known fact that the higher the mechanical properties of amaterial are (which is the case with HEL and VHEL steels) the less theneed for a thick hot-dip galvanised coating to avoid affecting itsfatigue resistance.

KITAGAWA's diagram illustrates, for a given steel, variations in themaximal stress before breaking after one million cycles of beingcyclically subjected to stress, as a function of the thickness of aconventional galvanised coating.

In one example, the maximal stress prior to the steel breaking was equalto 350 MPa in the untreated state and remained essentially constant forgalvanised coatings with a thickness of less than approximately 80 μmbefore decreasing sharply.

Consequently, for the steel thus analysed, the maximum admissiblethickness of the galvanised coating was approximately 80 μm.

In order to characterise what effect a galvanised coating has on thefatigue resistance of different HEL or VHEL steel compositions denotedby A to E, the maximal stress before break was determined after thesesteels had been subjected to cyclical stress of one million cycles inthe untreated state and coated with a galvanised coating.

The percentage loss of fatigue resistance was then calculated for theuntreated samples and the coated samples, and the maximum thickness ofthe galvanised coating without affecting fatigue resistance was definedon the basis of the KITAGAWA diagram.

The results are set out in Table 1 below. Maximal Maximal % loss inLimit of stress at stress at fatigue coating break after break afterresistance thickness HEL or Coating 1-10⁶ cycles 1-10⁶ cycles afteraccording VHEL thickness for untreated for coated 1-10⁶ to steels (μm)steel steel cycles Kitagawa A 61.5 μm   / / 0 >80 μm  B 65 μm 380 MPa380 MPa 0 80 μm C 63 μm 440 MPa 422 MPa −5% 60 μm D 65 μm 460 MPa 420MPa −8% 55 μm E 72 μm 525 μm  400 MPa −23%  50 μm

Accordingly, it was observed that in the case of certain steelcompositions, there was no loss of fatigue resistance after one millioncycles, which means that the galvanised coating did not affect themechanical characteristics of the steel (for example steel B), whereasin the case of other steel compositions, such as steel E for example,there may be a loss of more than 20% in fatigue resistance in thepresence of a galvanised coating with a thickness of 72 μm, which meansthat a maximum thickness of 50 μm must not be exceeded.

A comparative test for fatigue resistance was run in parallel on asample of VHEL steel coated with a conventional galvanised coating(Galva A) and a galvanised coating produced following treatment in abath proposed by the invention (Invention).

The results are set out in Table 2 below. State of the Maximal stress at% loss of fatigue material break Coating thickness resistance Untreatedstate 480 MPA None / Galva A 400 MPa 40 μm 20% Invention 450 MPa 40 μm 7%

It was found that for an identical coating thickness, the loss infatigue resistance compared with the untreated, non-coated sample was20% for the Galva A sample compared with only 7% in the case of thattreated in the galvanisation bath proposed by the invention.

This result proves that the galvanisation bath proposed by the inventionenables a specific structure to be deposited, of a type conducive tolimiting the drop in the steel's fatigue resistance.

The particularly advantageous nature of the galvanisation bath proposedby the invention was also demonstrated by the example described below.

18 samples of steel with a variable content of silicon and phosphoruswere prepared.

The compositions of these steels are specified in Table 3 below. SteelChemical composition by weight (%) Si equi- No. SI P C Mn S Al Ni Tivalent 1 0.010 0.008 0.070 0.310 0.004 0.030 0.030 2 0.236 0.008 0.2261.143 0.003 0.039 0.018 0.035 0.256 3 0.013 0.011 0.055 0.342 0.0270.003 0.041 4 0.013 0.017 0.082 1.452 0.005 0.029 0.040 5 0.056 0.0170.130 1.155 0.002 0.031 0.099 6 0.365 0.018 0.113 1.395 0.002 0.0400.410 7 0.207 0.016 0.141 1.916 0.001 0.024 0.247 8 1.707 0.020 0.2261.654 0.004 0.043 0.020 0.004 1.756 9 <0.01 0.017 0.087 1.570 0.0040.039 0.0425 10 0.210 0.010 0.120 1.500 0.004 0.029 0.090 0.002 0.235 110.220 0.013 0.240 1.210 0.003 0.042 0.030 0.033 0.253 12 0.010 0.0080.050 0.200 0.003 0.039 0.040 0.017 0.030 13 0.350 0.009 0.056 0.6300.003 0.039 0.020 0.003 0.372 14 <0.01 0.011 0.028 15 0.063 0.014 0.09816 0.061 0.012 0.122 1.448 0.002 1.370 0.021 0.005 0.092 17 0.328 0.0080.121 1.274 0.013 0.040 0.024 0.349 18 0.663 0.015 0.149 1.891 0.0030.047 0.030 0.112 0.700

These 18 samples were subjected to a conventional prior degreasing,rinsing, acid dipping, fluxing and stoving treatment.

They were then immersed for 7 minutes in a galvanisation bath proposedby the invention, heated to a temperature of 450° C. and containing0.10% of vanadium, 0.17% of manganese, 0.2% of bismuth, 0.2% of tin and0.0060% of aluminium, the rest making up the 100% being zinc ofcommercial purity.

The hot-dip galvanised coatings thus obtained were analysed and inparticular their mean thicknesses and weights calculated.

The characteristics of these coatings are set out in Table 4 below.Thickness (μm) Coating Appear- Delta weight ance of Steel n° mean minimaxi (maxi-mini) Diff. (g/m²) coating 1 59.3 48.8 70.8 22.0 5.7 472Satin 2 70.8 64.6 76.8 12.2 4.2 523 (qq grains) satin 3 56.4 46.9 75.828.9 8.3 428 (qq grains) satin 4 56.9 49.2 63.3 14.1 4.4 437 satin 560.1 50.8 66.0 15.2 4.6 435 satin 6 58.9 50.9 66.4 15.5 3.9 478 satin 775.1 67.9 80.6 12.7 3.7 519 satin 8 71.5 61.1 78.7 17.6 6.0 528 satin 955.9 52.3 60.9 8.6 2.7 411 satin 10 66.0 57.7 77.8 20.1 6.2 469 satin 1170.5 63.7 76.1 12.4 3.5 — satin 12 58.9 55.4 68.2 12.8 4.1 426 (qqgrains) satin 13 63.8 58.3 74.0 15.7 4.6 454 satin 14 53.8 45.3 65.420.1 5.6 387 (qq grains) satin 15 57.3 48.5 61.9 13.4 4.2 403 satin 1656.9 48.6 64.0 15.4 5.2 — satin 17 64.2 56.3 67.5 11.2 3.3 467 satin 1866.6 60.0 71.8 11.8 3.3 476 satin

Curves were also plotted for the variations in mean thickness (FIG. 2)and weight (FIG. 3) of the galvanised coatings as a function of thecontent of Si equivalent (Si+2.5P) for the steel samples.

These curves incontestably show that the galvanisation bath proposed bythe invention enables the Sandelin curve to be “smoothed”, therebyproducing satisfactory galvanised coatings irrespective of thecomposition of the steel sample.

1) Hot-dip galvanisation bath for parts made from any steel composition,in particular parts made from steel containing silicon and/orphosphorus, which have undergone a prior degreasing, acid dipping andfluxing treatment, characterised in that it is made up of a zinc alloycontaining: 0.1 to 1.5% by weight of bismuth, 0.1 to 1.5% by weight oftin, 0.04 to 0.15% by weight of vanadium, 0.10 to 0.30% by weight ofmanganese and at least 0.002% by weight of aluminium, the rest making upthe 100% by weight being zinc of commercial purity. 2) Hot-dipgalvanisation bath as claimed in claim 1, characterised in that itcontains between 0.06 and 0.12% by weight of vanadium. 3) Hot-dipgalvanisation bath as claimed in claim 1, characterised in that itcontains between 0.15 and 0.25% by weight of manganese. 4) Hot-dipgalvanisation bath as claimed in claim 1, characterised in that itcontains between 0.0040 and 0.20% by weight of aluminium. 5) Hot-dipgalvanisation bath as claimed in claim 1, characterised in that itcontains, as a proportion by weight, between 0.5 and 1.5% of bismuth,between 0.5 and 1.5% of tin, between 0.06 and 0.12% of vanadium, between0.15 and 0.25% of manganese and between 0.004 and 0.020% of aluminium.6) Hot-dip galvanisation bath as claimed in claim 1, characterised inthat it contains, as a proportion by weight, 0.10% of vanadium, 0.17% ofmanganese, 0.006% of aluminium, 0.2% of tin and 0.2% of bismuth.